The good news is you can live to be over 120 in China. The bad news is that you have to be a peasant living in poverty on top of a mountain in possibly the poorest county in China. And you have to eat your greens.

'My advice is this: work hard, be happy, eat maize every day and plenty of fresh greens,' said Mrs Wang Yunzhe who, at the age of 107, still braids her hair in a pigtail and is spritely enough to do the cooking.

She is only one of the many centenarians in Bama county, a remote spot in the interior of Guangxi province, which borders Vietnam. Quite why people should live so long in Bama is the question Dr Chen Jingchao, director of the Bama County Longevity Research Centre, has been trying to answer.

According to Dr Chen, the oldest person in the district - and probably in all China - is Luo Mazhen. He is 128, some years older than the French woman officially recognised as the oldest person in the world. While there is no documentary proof of Mr Luo's birth, Dr Chen has found records of the birth of his grandson, now aged 78.

Mr Luo was not available for interview, as he still lives and farms in a remote settlement in Suolou village, six hours' walk from the nearest road. Bama county, which is itself a day's drive from the provincial capital of Nanning, has the typically rugged and picturesque landscape of southwest China. As in Guilin, dazzling rivers of ultramarine flow among the curious domed mountains and terraced valleys of brilliant green.

In these isolated hinterlands, a mixed population of Miao, Dao, Zhuang and Han clans struggle to make a living farming some of the worst land in the country. Mr Luo belongs to the Dao minority, as did his friend Nan Mekun, who died at the age of 131 four years ago.

But Dr Chen believes longevity has nothing to do with race. Mrs Wang is a Han Chinese and other centenarians are members of the Zhuang clan.

'This place has been studied since the 1950s and there is no one factor to account for longevity. It is to do with the diet, the pure air, the gentle climate, the clean water and the absence of all pollutants,' he said.

To demonstrate the purity and simplicity of life in Bama, Dr Chen took me to see Huang Meihun, a 105-year-old. She is a Zhuang and lives with 65 of her descendants in a crudely built cottage.

Dressed in ragged black clothes, she believes the key to a long life is peace and calm. She does not worry about money because nobody has any - most peasants earn less than $400 a year - and she eats what she grows: beans and maize. People in the region rarely eat meat. Mrs Huang never consumes tea, oil or sugar.

'Sometimes I drink a little maize wine which we make,' she confessed. But that is her only vice. The pressures of the modern world seem a long way away. Television arrived three years ago, but she says it does not interest her much.

Dr Chen admits Mrs Huang provides proof of a recent theory that longevity has to do with surviving on a reduced diet. For most of her life, Mrs Huang has consumed few of the kind of calories often recommended by nutritionists in the West.

Ji Yang, another researcher into longevity in Bama, has concluded that it helps to be female. Of the 289 inhabitants aged over 90, 188 are women - and virtuous ones at that.

'In Bama, it is the custom for young people to start their love affairs at the age of 15 or 16, when they are able to meet desirable friends of the opposite sex through social activities such as singing.

'But marriage and childbirth will not take place until more mature ages,' he says. A couple's first child is generally born when the woman is in her mid to late 20s.

Dr Chen and the government of Bama are quick to promote the exceptional and invigorating qualities of their environment. Bama's reputation for longevity will, they hope, provide the key to escaping poverty.

Given the ancient Chinese preoccupation with finding an elixir of immortality, tens of thousands of seekers are now making their way there. Last year there were 100,000 visitors. Throughout the county, precious paddy fields are being converted into guest houses.

'Lots of top leaders like Li Peng , Zhu Rongji , and Chen Yun's widow have all come here,' Dr Chen said. His job is now to protect the environment and its magical qualities from the deadly effects of modern life.

To create jobs, the county fathers are trying to raise investment to expand one of its few factories, a brewery making a longevity wine.

'The wine is based on the ancient recipes of the locals,' said Shen Jiquan, the brewery's former director. The ordinary stuff is brewed from 30 wild herbs, but there is also a special wine, allegedly imbibed by China's Olympic team, and made with local vipers. In the factory, he proudly lifts the lid on a vat of fermenting snakes.

And the county is investing in a new factory to make special longevity medicines, which are based on a secret recipe of hemp, maize, tea oil, beans and various other parts of the local diet.

Maize, one of the few crops to flourish in the local soil, is used to make Bama Long Life Crisps. A Hong Kong company is investing in a bottling plant to market the local spring water.

For centenarians like Mrs Wang, there are no regrets that the busy world outside has, for the time being at least, passed Bama by.

She remembers that Emperor Guangxu was on the throne when she was born and he was followed by Xuan Tong. She vaguely remembers the communist-led rebellion which Deng Xiaoping tried to ferment in the region in 1929, and the invasion by Japan, but confessed she had not followed more recent events.

'Deng Xiaoping?' she said, 'I remember hearing about him in the late 1920s. I know he was here but I never met him. I don't know what happened to him after that.'